In collaborative studies with the Center for Research Education, V.A. Hospital, Los Angeles, the histamine-containing cell in dog and rat gastric mucosa has been identified as a small mast with a histamine content of around 3 pg/cell. Histidine decarboxylase activity is also present in this cell in rat and, in smaller amounts, in dog. The cell exhibits the characteristic metachromasia upon staining with toluidine blue. Unlike the mast cell, it is resistant to the histamine-depleting action of Compound 48/80 and has no IgE receptors. Fractionation of other mucosal cells by centrifugal elutriation has further shown that serotonin and DOPA decarboxylase reside exclusively in enterochromaffin cells. The enzymes responsible for the degradation of these amines are present in different but unidentified types of cells.